1. Field of the Invention
This invention relates broadly to analog-to-digital converters. More particularly, this invention relates to analog-to-digital converters that store an analog input signal voltage level on one or more capacitive circuit elements.
2. State of the Art
As shown in FIG. 1, an analog-to-digital converter 10 typically includes a sample-and-hold input stage that utilizes a switch 12 to selectively couple a sampling capacitor Csample to the input 14, which allows the sampling capacitor Csample to charge to an analog voltage level that corresponds to the analog voltage level at the input 14. The switch 12 is opened and analog-to-digital conversion circuitry 16 converts the analog voltage level stored on the sampling capacitor Csample to a sequence of digital words (each word being one or more bits) corresponding thereto. As is well known in the electronic arts, there are many different architectures that may be used to implement the analog-to-digital conversion process carried out by the converter 16, including voltage-scaling converters, charge-scaling converters, successive-approximation converters, flash-type converters, sigma-delta converters, etc. The suitability of a given approach is dependent upon the desired resolution (e.g., number of bits) of the conversion process and the frequency of the signal that is converted.
Conventionally, an operational amplifier buffer 18 is used to charge the sampling capacitor Csample to an analog voltage level that corresponds to the analog voltage level at the input 14. This configuration is shown in FIG. 1. The output impedance of the operational amplifier 18 together with the sampling capacitor Csample cause a lagging phase shift. This phase shift, together with the operational amplifier's internal phase shifts and high feedback ratio, causes undesired instability and/or ringing. Such instability and/or ringing is exacerbated by the fact that the sampling capacitor Csample is switchably coupled to the output of the operational amplifier 18 at the sampling rate of the analog-to-digital converter 16 (or a multiple thereof). In audio applications, such instability and/or ringing distorts the audio input signal and subsequent analog-to-digital conversion process, which is typically most evident at high audio frequencies.
In addition, any “real world” operational amplifier will have an input offset voltage error (typically due to internal device mismatches or other circuit imbalances). Such input offset voltage errors, if significant, will make the operational amplifier 18 unsuitable for use in multi-bit analog-to-digital conversion operations. Moreover, “real world” operational amplifiers with low input offset voltage errors generally have insufficient slew rates to achieve low distortion at high audio frequencies.
Thus, there is a need in the art to provide improved circuitry that converts an analog audio signal into digital form in a manner that reduces distortion in the conversion process.